Story highlights

One girl, 17, charged as an adult. Other girl is 15.

They face assault, pornography charges

Sheriff's spokeswoman says sex acts were recorded on cell phones

Two Maryland teenage girls were charged this week with assaulting an autistic boy and recording him on their cell phones performing sex acts, including some with animals, the St. Mary County Sheriff's Department said Wednesday.

Authorities said the two girls, ages 17 and 15, admitted to police that they assaulted the boy, 16, with a knife on "multiple occasions."

"His parents refer to him as (having) been diagnosed autistic," said Sgt. Cara Grumbles, a spokeswoman for the sheriff's office. "Some of the sexual acts involved animals. It's awful."

A news release from the department said the girls also kicked the boy in the groin and dragged him by his hair. They also got him to walk onto a partially frozen pond. When the boy fell through several times, they made him get out each time by himself, police said.